Lithium batteries which employ Li as the negative active material can constitute high-voltage battery systems by combining with an appropriate positive active material, and the energy density, especially weight efficiency (Wh/kg), can be improved. For this reason, various types of lithium batteries are put into practical use in a large volume as small, light-weight power sources for portable equipment.
In addition to lithium primary batteries which need to be replaced after end of discharge, lithium secondary batteries which can be renewed for continued use have been developed and are becoming commercially practical.
Since metallic Li reacts with water generating hydrogen, non-aqueous electrolyte such as liquid organic electrolyte obtained by dissolving a certain kind of Li salt into a dehydrated aprotic organic solvent or solid polymer electrolyte is generally used as the electrolyte in lithium batteries.
If metallic Li can be used as it is as a negative electrode material for a secondary battery as in a primary battery, the negative electrode potential becomes the least noble making it possible to construct a high-voltage battery system and providing advantage from the standpoint of energy density. However, metallic Li negative electrode suffers the problem of causing active dendritic or mossy crystals of metallic Li to deposit on the negative electrode surface during charge, which penetrate the separator and tend to cause internal short circuit with the positive electrode, thus making it unable to achieve a long cycle life.
Furthermore, the deposited dendritic or mossy crystals of metallic Li react with the solvent in the organic electrolyte making it inactive, thus making the battery unrenewable by charge and resulting in a relative decrease of the capacity. It was therefore necessary in designing and manufacturing batteries to load a large quantity of the negative active material in anticipation of such a decrease of capacity, indicating that it is not necessarily a good negative electrode material appropriate for realizing a high capacity.
In order to suppress such deposition of dendritic or mossy crystals during charge, alloys of Li with aluminum or with Wood's metal which is a fusible alloy have been tried as a negative electrode material. A negative electrode comprising such metals as can make alloys with Li or Li alloys containing at least one of such metals shows a relatively high capacity in the initial cycles of charge-discharge. However, through repetition of alloying with Li due to charge and detachment of Li due to discharge, a phase different from the original one is caused though keeping the original crystal structure of the skeleton alloy, or a change into a crystal structure which is different from the original skeleton alloy tended to be caused.
Through such a phenomenon, crystal grains of the metal of the alloy acting as the host material of Li active material undergo swelling and shrinking, and as the charge-discharge cycles the progresses, cracks take place in the crystal grains of the metal or alloy as the host material resulting in the grains becoming fine. Such phenomenon of the grains becoming fine causes an increase in the ohmic resistance among grains of the negative electrode material, and deterioration of the charge-discharge characteristic due to an increase in the resistance polarization during charge and discharge. Consequently, the use of a negative electrode material comprising a Li alloy is currently limited to the negative electrode material for lithium secondary batteries for applications such as memory backup which is not always subjected to a deep discharge.
On the other hand, a system in which a carbon material such as graphite capable of repeating absorption and desorption of lithium ions (Li.sup.+) with charge and discharge is employed as the host material in the negative electrode material has been recently commercialized under the name of lithium-ion secondary batteries. As the positive electrode material, lithium cobaltate (LiCoO.sub.2), lithium nickelate (LiNiO.sub.2), or spinel-type lithium manganese oxide (LiMn.sub.2 O.sub.4), which are all capable of repeating desorption and absorption of lithium ions (Li.sup.+) with charge and discharge, are used similarly to the negative electrode. As the lithium-ion secondary batteries have a long cycle life, increasingly more of them are being used in small and light weight power sources for portable telephones, camcorders, and notebook type personal computers.
The capacity (mAh) and energy density (mWh.multidot.g.sup.-1) of lithium-ion secondary batteries have a close interrelationship primarily with the capacity density (mAh.multidot.g.sup.-1) of the carbon material used as the host material of Li.sup.+ of the negative electrode. As is well known, carbon has a wide range of forms from crystalline graphite to amorphous carbon and its characteristic as a negative electrode material is heavily dependent on its physical properties.
As an example, use in a negative electrode of graphitic carbon material made from a material generally referred to as graphitizable carbon or soft carbon is disclosed in Japanese Laid-Open Patent No. Sho 60-182670 and Japanese Laid-Open Patent No. Hei 4-155776. When using graphite, the theoretical capacity of its carbon material is calculated to be 372 mAh.multidot.g.sup.-1 from Eqn. (1). ##STR1##
When using a graphite group carbon material in the negative electrode, a capacity close to the theoretical capacity is obtained and, as the charge-discharge potential is approximately equal to the dissolution and deposition potential of metallic Li and is extremely flat, a lithium-ion battery having a high capacity and with a stable voltage can be realized. However, graphite group material with a high degree of crystallization has a drawback of decomposing organic solvent of the liquid electrolyte.
In contrast to this, use of an amorphous or low-crystallization carbon material generally referred to as non-graphitizable carbon or hard carbon in the negative electrode is disclosed in Japanese Laid-Open Patent No. Sho 61-111907 and Japanese Laid-Open Patent No. Sho 62-90863. In these cases, though the flatness of the discharge voltage as a lithium-ion secondary battery is inferior, it has the features of suppressing decomposition of organic solvent of the electrolyte and at the same time achieving a high capacity in excess of the theoretical capacity of graphite or graphitic carbon materials of 372 mAh.multidot.g.sup.-1.
In order to achieve a high capacity of a carbon material for negative electrode in a lithium-ion secondary battery, it is necessary that a large quantity of Li be absorbed by insertion into the carbon material in the initial charge and that a large discharge capacity be taken out in the initial discharge. Usually, however, the above-mentioned initial discharge capacity is smaller than the initial charge capacity with some inactive irreversible capacity existing due to dead lithium absorbed and fixed within the carbon material without contributing to subsequent discharge. Though conventional soft carbon is suitable for achieving a higher capacity, it also suffers a serious drawback of having a large irreversible capacity. Lithium-ion secondary batteries suffer a problem of the liquid organic electrolyte being decomposed or internal short-circuit caused when subjected to over charge. The greater the irreversible capacity of the carbon material of the negative electrode is, the more tho positive electrode is over charged and decomposes liquid organic electrolyte. Accordingly, when constructing a battery, it is necessary to increase the positive electrode capacity in the amount equal to the irreversible capacity of the negative electrode in order to suppress over charge. This increment of the positive electrode capacity equal to the irreversible capacity of the negative electrode is an inefficient portion which cannot contribute to subsequent discharge and is a limiting factor in achieving a higher capacity.
Consequently, it is an extremely significant requirement that the negative electrode material to obtain a high-capacity lithium-ion secondary battery should have a characteristic of absorbing a large quantity of lithium in the initial charge while having a small irreversible capacity.